This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-179358 filed on Jun. 9, 2000.
1. Field of the Invention
The present invention relates to a variable valve timing control device suitable foe use in an internal combustion engine.
2. Description of Related Art
There has been an increasing tendency that a vehicle-mounted internal combustion engine uses a variable valve timing device for the purpose of improving engine output, improving fuel economy, and reducing exhaust emissions. Most variable valve timing devices in practical use are designed to control the advance amount of the intake valve timing to advance the intake valve timing during part-load operation, thereby increasing the amount of valve overlap to increase the amount of internal EGR gases (amount of residual gases) and accordingly decreasing a pumping loss and improving fuel economy.
However, during low-load operation in which an intake air amount is small, when the amount of valve overlap is increased to increase the amount of internal EGR gases, intake air into engine cylinders is disturbed by blow by of residual gases to the intake side, thereby resulting in lowered fuel combustion and worsening exhaust emission. Furthermore engine vibration increases to worsen drivability. Thus, in a conventional system, it is necessary to reduce the internal EGR gases for combustion stability during low-load operation. In this case, there exists a disadvantage that the operation range within which the fuel economy is improved due to the internal EGR is limited to a medium or greater load, thereby lowering fuel economy improving effect.
An object of the present invention is to provide a variable valve timing control device for an internal combustion engine which improves fuel consumption ratio by internal EGR without worsening the state of fuel combustion even during low-load operation, and accordingly improve all of the fuel economy, exhaust emissions, and drivability.
In the conventional internal EGR by valve overlap, residual gases in the cylinders (internal EGR gases) during the valve overlap are with the intake air, thereby lowering the cylinder temperature and worsening fuel combustion efficiency.
By taking this point into account, according to a first aspect of the present invention, an exhaust valve advanced-closing control for controlling the valve closing timing of the exhaust valve to the advance side from an intake TDC is executed without using valve overlap by means of a valve timing control means. As in the present invention, when the exhaust valve is closed at an earlier timing than the intake TDC, residual combustion gases in the cylinder is trapped inside of the cylinder, and becomes internal EGR gases. At this time, since the intake valve is held in a closed position for some time after the trapping of the residual gases, it is possible to prevent a temperature drop of the residual gases due to the introduction of the intake air unlike the internal EGR by conventional valve overlap, and also is possible to compress by a piston the residual gases in the cylinder to raise the residual gas temperature during a period from the valve closing timing of the exhaust valve till the intake TDC (or the timing to open the intake valve). Therefore, according to the present invention, even when the internal EGR (the exhaust valve advanced-closing control) is executed during low-load operation, the intake air can be taken into the cylinder with the cylinder temperature held higher than that of the conventional internal EGR, thereby improving fuel atomization in the cylinder to stabilize the state of combustion. Consequently, even when the internal EGR (the exhaust valve advanced-closing control) is executed during low-load operation, it is possible to prevent worsening of emissions due to poor fuel combustion, and furthermore to reduce engine vibration to improve drivability, thereby improving the fuel economy by the stabilization of combustion and the internal EGR.
According to the second aspect of the present invention, a changeover may be made, in accordance with engine load, between the exhaust valve advanced-closing control for controlling the timing to close the exhaust valve to the advance side of the intake TDC and the retarded exhaust valve closing control for controlling the timing to close the exhaust valve to the retard side thereof. For example, at the low-load region, the exhaust valve advanced-closing control is executed to improve fuel economy while stabilizing the state of combustion, and at over the load range, the retarded exhaust valve closing control is carried out to reduce a pumping loss to increase the engine output.